360 THE VERTEBRATE SKELETON. 



cervical vertebrae with the exception of the atlas almost 

 always have markedly opisthocoelous centra, but in Macrau- 

 chenia they are flat. The nasals are expanded posteriorly, and 

 an alisphenoid canal is present. The thoraco-lumbar vertebrae 

 are never less than twenty-two in number and are usually 

 twenty-three. The femur has a third trochanter (except in 

 Chalicotherium). The third digit of the manus and pes is 

 symmetrical in itself, and larger than the others, and in some 

 cases the other digits are quite vestigial. The number of the 

 digits of the pes is always odd. The astragalus is abruptly 

 truncated distally, and the facet by which it articulates with 

 the cuboid, is much smaller than that by which it articulates 

 with the navicular. The calcaneum does not articulate with 

 the fibula, except in Macrauchenia, The group includes many 

 extinct forms, and the living families of the Tapirs, Horses 

 and Asses, and Rhinoceroses. 



Section II. SUBUNGULATA. 



In this group is placed a heterogeneous collection of 

 animals, the great majority of which are extinct. There is 

 really no characteristic which is common to them all, and 

 which serves to distinguish them as a group from the Ungulata 

 vera. But the most distinctive character common to the 

 greatest number of them is to be found in the carpus, whose 

 bones in most cases retain their primitive relation to one 

 another, the os magnum articulating with the lunar and some- 

 times just meeting the cuneiform, but in living forms at any 

 rate not articulating with the scaphoid. The feet frequently 

 have five functional digits, and may be plantigrade. The 

 proximal surface of the astragalus is generally flattened in- 

 stead of being pulley -like as in Ungulata vera. 



